There is a method of supplying fuel gas to a fuel cell in which a liquid raw fuel that contains hydrocarbons such as methanol and gasoline is reformed into hydrogen rich fuel gas (referred to below as hydrogen rich gas) by a reforming system, and this hydrogen rich gas is supplied as fuel gas to a fuel cell (see, for example, Japanese Unexamined Patent Application, First Publication No. 2001-132909).
In this reforming system, a liquid fuel (i.e., a raw material) that is formed by a mixture of raw fuel and water is vaporized by a vaporizer to form raw fuel gas. This raw fuel gas is then supplied to a reformer together with air for reforming, and the raw fuel gas is made to undergo a reforming reaction so as to be reformed into hydrogen rich gas.
The conventional vaporizer that is used in this reforming system is disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-135331.
This vaporizer is provided with a catalytic combustor in which an oxidation catalyst (for example, Pt or Pd) is supported on a metal honeycomb support, a group of tubes that are bent substantially in a U-shape and that circulate combustion gas generated by the catalytic combustor, a vaporization chamber that contains the group of tubes and is surrounded by a shell, a fuel supply apparatus that injects the liquid fuel into the vaporization chamber, and an extraction aperture that conducts the raw fuel gas generated by the vaporization of the liquid fuel out of the vaporization chamber. In this vaporizer, off gas that is discharged from the anode or cathode of the fuel cell is catalytically combusted by the catalytic combustor, and the combustion gas that is thereby obtained is introduced into the group of tubes. At the same time as this, the liquid fuel from the fuel supply apparatus is injected towards the surface of the group of tubes, and a thermal exchange takes place between the combustion gas and the liquid fuel, so that the liquid fuel is vaporized and raw fuel gas is obtained.
However, in this conventional vaporizer, in some cases a portion of the liquid fuel that is supplied to the vaporization chamber from the fuel supply apparatus is unable to be vaporized as it passes through the group of tubes, and is left on the bottom of the vaporization chamber in a liquid state. If liquid fuel is left in a liquid state on the bottom of the vaporization chamber in this manner, then it is not possible for the quantity of generated vapor (i.e., the quantity of generated raw fuel gas) during a transition to match the output from the fuel cell, so that the problem arises that the response is deteriorated.
Moreover, even if a heating source such as a catalytic combustor is placed on the bottom of the vaporization chamber so as to reduce the quantity of liquid left behind, it is still difficult to obtain a satisfactory response. This applies even more particularly in a fuel reforming system for a fuel cell that is mounted in a fuel cell vehicle, in which an extremely high transition response is required.
In Japanese Unexamined Patent Application, First Publication No. 2001-332283, a vaporizer is disclosed in which vaporization flow paths through which liquid fuel and fuel vapor circulate are placed alternatingly with heating gas flow paths through which heating gas circulates. Liquid fuel is supplied from above the vaporization flow path and drops down the vaporization flow path, and the liquid fuel is vaporized by a thermal exchange with the heating gas as it drops. As a result, fuel vapor is created, and the created fuel vapor is discharged from the bottom of the vaporization flow path.
However, in this vaporizer, because the fuel vapor flows downwards in the gravitational direction inside the vaporization flow path so as to be in a parallel flow relationship with the liquid fuel, it is not possible to effectively utilize the energy possessed by the fuel vapor.
Furthermore, in a conventional vaporizer, because combustion gas is introduced into a large number of tubes that have been bent substantially in a U-shape and is circulated through these tubes, there is a considerable pressure loss in the fuel gas. Accordingly, in order to increase the fuel gas flow rate so as to increase the heat quantity, it has been necessary to increase the number of tubes, thereby creating the problem that the size of the evaporator has increased. In other words, the size of the pressure loss on the fuel gas flow path is an obstacle to reducing the size of the vaporizer.
Moreover, when a combustion catalyst is supported inside the tubes, it is not possible to secure a sufficient catalytic layer volume.
Note that, in the above description, the term “response” is a characteristic that expresses a temporal delay when vapor is generated and supplied to match the required quantity that is determined based on a varying output from a fuel cell, and a “poor response” refers to the fact that this delay is sizeable. In addition, the term “a transition” refers to a state in which the quantity of generated vapor that is required relative to the output from the fuel cell abruptly changes, and “transition response” refers to the response during a transition.